The present invention relates to a vibration isolation apparatus and, more particularly, to an arrangement of a sensor in a vibration isolation apparatus.
In order to control transmission of shock or vibration between a sprung member and an unsprung member, there is a need to arrange a load sensor in a vibration isolation apparatus, such as a vehicle suspension unit.
Published Japanese Utility Model Application No. 62-43909 shows an arrangement of a load sensor in a vibration isolation apparatus (see FIG. 10 of this publication). This known vibration isolation apparatus is coupled in a force transmitting relationship between a sprung member in the form of a vehicle axle and an unsprung member in the form of a vehicle body, and includes as a suspension member a piston rod having a lower end inserted into a cylinder and an upper end inserted through an inner bracket of an upper mount insulator which has its outer bracket fixed to the vehicle body. An annular piezoelectric sensing element is coupled with the piston rod and set in position by tightening a nut for fixedly coupling the piston rod with the inner bracket of the upper insulator.
This known sensor arrangement using the piezoelectric sensing element has the following problems:
Initial stress to which the sensing element is subjected to is determined by a torque for tightening the nut. With this arrangement, the initial stress applied to the sensing element becomes substantially great. This causes a difficulty in enhancing the measurement accuracy since the maximum peak strain of the sensing element becomes excessively high. Further, since the sensing element is required to have a rigidity and strength high enough to withstand the substantial initial stress, it is difficult to increase a magnification of a strain relative to an input load. Thus, there is a limit to measurement accuracy. Regarding the rigidity and strength of the sensing element required, the sensing element must be rigid and strong enough to withstand an input load induced upon impact with a bound stop of the vibration isolation apparatus. Thus, enhancement of the measurement accuracy is hardly expected.
With this arrangement wherein the initial stress applied to the sensing element is determined by the nut tightening torque, since it is very difficult to control the nut tightening torque during the production line, a product-to-product variability of the initial stress of the sensing element occurs. There is a difference in thermal expansion between the sensing element and the piston rod, causing a variation in the nut tightening torque which in turn induces a variation in transmission of input load to the sensing element. Since the sensing element is coupled with the piston rod, the sensing element is inclined when the piston rod is subject to a bending moment or stress.
An object of the present invention is to provide a sensor arrangement of a vibration isolation apparatus which is free from the above-mentioned problems which are obstacles to enhancement of measurement accuracy.